The invention relates to hydrostatic transmissions. More specifically, the field of the invention is that of expander bands used in radial piston hydraulic pumps and motors. Where a radial piston hydraulic pump is fluidly coupled to a radial hydraulic motor, it is known by the name hydrostatic transmission.
A hydrostatic transmission transmits energy from mechanical rotary motion to fluid (typically oil) movement, then back to mechanical rotary motion. Within a transmission, a cylinder unit is eccentric to and rotates on a pintle, with pistons positioned within the cylinders and attached to slippers mounted in an expander band, so that as the cylinder unit rotates the pistons are driven by the slippers which engage the surrounding eccentric annular track ring. The pistons of the driving cylinder unit create a pressurized fluid flow that drives a receiving set of pistons which drive an output shaft. The transmission ratio is directly proportional to the eccentricity of the track ring relative to the pintle. U.S. Pat. Nos. 4,635,535 and 4,691,512, which are incorporated by reference herein, describe hydraulic units which may be used within a hydrostatic transmission.
One conventional design of an expander band includes a plurality of slots circumferentially disposed at approximately equal distances. The expander band slots allow the slippers to migrate from one end of the slot to the other as the cylinder unit rotates, preventing the hydraulic pump or motor from binding up. Unfortunately, the slippers tend to impact against and abrade the ends of the slots, which causes wear on the ends of the slots and slippers. This wear can affect pump performance and require repair or replacement. In addition, the ends can chip away, causing metal particles to be introduced in the oil. Debris in the circulating oil can limit the efficiency and life of the pump, and in the worst case cause it to fail.
What is needed in the art is an expander band assembly for a hydrostatic transmission which minimizes wear and chipping. Also needed is an expander band assembly which is simple and economical for use within a hydrostatic transmission. A further need exists for an expander band assembly which positions the slippers without binding.